Pompe disease also known as acid maltase deficiency or Glycogen storage disease type II is an autosomal recessive metabolic disorder which damages muscle and nerve cells throughout the body. It is caused by an accumulation of glycogen in the lysosome due to a deficiency of the lysosomal acid alpha-glucosidase enzyme. The build-up of glycogen causes progressive muscle weakness (myopathy) throughout the body and affects various body tissues, particularly in the heart, skeletal muscles, liver and nervous system.
In Pompe disease, a protein, acid alpha-glucosidase (EC 3.2.1.20), also known as acid maltase, which is a lysosomal hydrolase, is defective. The protein is an enzyme that normally degrades the alpha-1,4 and alpha-1,6 linkages in glycogen, maltose and isomaltose and is required for the degradation of 1-3% of cellular glycogen. The deficiency of this enzyme results in the accumulation of structurally normal glycogen in lysosomes and cytoplasm in affected individuals. Excessive glycogen storage within lysosomes may interrupt normal functioning of other organelles and lead to cellular injury. The defective protein is the result of alternative splicing which is caused by mutations in the GAA gene on long arm of chromosome 17 at 17q25.2-q25.3 (base pair 75,689,876 to 75,708,272). The gene spans approximately 20 kb and contains 20 exons with the first exon being noncoding.
Although over 460 GAA mutations have been described (http://cluster15.erasmusmc.nl/klgn/pompe/mutations.html), only a few splicing mutations have been characterized. Severe mutations that completely abrogate GAA enzyme activity cause a classic infantile disease course with hypertrophic cardiomyopathy, general skeletal muscle weakness, and respiratory failure and result in death within 1.5 years of life. Milder mutations leave partial GAA enzyme activity and results in a milder phenotype with onset varying from childhood to adult. In general, a higher residual enzyme activity in primary fibroblasts is associated with later onset of Pompe disease. Enzyme replacement therapy (ERT) has been developed for Pompe disease, in which recombinant human GAA protein is administered intravenously every two weeks. This treatment can rescue the lives of classic infantile patients and delay disease progression of later onset patients, but the effects are heterogeneous. The IVS1 mutation, c.-32-13T>G, a transversion (T to G) mutation that is the most common among children, juveniles and adults with this disorder. This mutation interrupts a site of RNA splicing.
Antisense oligonucleotides (antisense oligomeric compounds) are currently being tested in clinical trials for their ability to modulate splicing. A classical example is Duchenne muscular dystrophy. In this disease, mutation hotspots are present in certain exons. Using antisense oligomeric compounds, the mutated exon is skipped and the mutation is bypassed. This results in a slightly shorter protein that is still partial functional. It is straightforward to induce exon skipping using antisense oligomeric compounds, because it is evident that the antisense oligomeric compound must be targeted to the relevant splice site. Also in Epidermolysis bullosa (WO2013053819) and in Leber congenital amaurosis symptoms (WO2012168435) antisense oligonucleotides are used for exon skipping.
For the IVS1 mutation in Pompe, such a strategy does not work. The IVS mutation causes a skipping of exon 2 resulting in the deletion of the canonical translation start side and leads to non-sense mediated decay and thus no protein is transcribed. For antisense therapy to work for the IVS1 mutation in Pompe disease, it needs to induce exon inclusion. However, it is very difficult to induce exon inclusion, because it relies on targeting a splicing repressor sequence, which cannot be reliably predicted. For the IVS1 mutation, an antisense oligomeric compound that blocks a splicing repressor sequence may promote exon 2 inclusion in the presence of the IVS1 mutation. It is known that such repressor sequences may be present anywhere in the gene, either in an exon (termed exonic splicing silencer or ESS) or in an intron (termed intronic splicing silencer or ISS) and maybe close to the mutation or far away or maybe close to the affected splice site or far away from it.
Although a number of antisense compounds that are capable of modulating splicing of a target gene in vitro have been reported, there remains a need to identify compounds that may modulate the splicing of the GAA gene.
It is therefore an object of the invention to provide an antisense compound that is capable of inducing exon inclusion. Another object of the invention is to provide an antisense compound that is capable of targeting exonic splicing silencer (ESS) or in an intronic splicing silencer (ISS). Yet another object of the invention is to provide a antisense compound that is capable of targeting the IVS-1 mutation. The present invention meets one or more of the objects.